Why do nebula begin to contract?

Nebulae begin to contract primarily due to gravitational forces. As gas and dust within the nebula clump together, their gravitational attraction pulls more material inward, increasing density and temperature. This process can be triggered by external factors, such as shock waves from nearby supernovae or interactions with other cosmic structures, which compress the material and initiate star formation. As the nebula contracts, it may eventually lead to the formation of new stars and planetary systems.